Objective: To study the mechanism of the selective transvascular passage of the mature myeloid cells from the extravascular site of formation into the intravascular pool, the possible changes or breakdown in this selective transvascular passage in leukemic states, the selective transvascular passage of lymphocytes (T-cells and B-cells) in the lymph node and the intraorgan transport of these lymphocytes in bone marrow, thymus and lymph node. Approach: Observations made by means by light microscopy, TEM and SEM on normal and leukemic hematopoietic organs. By means of immunohistochemical identification, the distribution of B- and T- lymphocytes in different hematopoietic organs is being studied. The transvascular passage and organ distribution of leukemic cells is being studied by means of tritium labeled malignant myeloid cells after intravascular administration.